Field of the Invention
The invention relates to a driver circuit.
Emitter followers are used as a rule to drive lines or capacitive loads, particularly in bipolar logic circuits. The emitter follower is quite suitable for reversing the charge of the line or capacitive load at the switching edge at which the emitter follower transistor is made conducting. Conversely, at the edge at which the emitter follower transistor is blocked, it is fast enough only when the bias current of the emitter follower is selected to be high enough. However, that current is actually needed only during the discharging operation of the load capacitor or of the line impedance. During the remainder of the time, it could also be equal to zero without any disadvantage. That would produce considerable savings of supply capacity.
Essentially two possible devices that make use of that concept are known. An article by George R. Watson, entitled "Advances in Bipolar VLSI", in Proceedings of the IEEE, Vol. 78, No. 11, November 1990, proposes turning on the bias current of the emitter follower during the edge that blocks the emitter follower transistor, through the use of a differentiation circuit. The on-time should be proportional to the magnitude of the load. However, that necessitates an adaptation to the prevailing load. Accordingly, that type of circuit is not very suitable for cell structures in customer-specific integrated circuits. Moreover, differentiation requires a capacitor which can only be constructed in integrated circuitry by using a comparatively great amount of surface area.
Another possible device contemplates using two switches in a manner similar to CMOS technology. The problem then is that the cross current or the quadrature-axis component of the current in the state of repose must be kept at a low value, regardless of the temperature and supply voltage. A certain bias current is nevertheless necessary, because otherwise the switching time would be reduced considerably. According to U.S. Pat. No. 5,381,057, that problem is solved in such a way that the switch draws the current from the charge reservoir of a capacitor and adjusts the bias current accordingly. A disadvantage then is that the capacitance of the capacitor must be dimensioned to suit the frequency of the switching and the number of drivers, and as a rule a comparatively large capacitor is therefore necessary. However, a large capacitor can only be integrated with difficulty, if at all.